Pneumatic machines having reciprocating air-powered motors have been used for rock drilling underground for many years. The machines are efficient, readily transportable, rugged in construction, and long-lived in operation. However, use of such machines underground has always been attended by a number of problems. Thus the operating air always contains some dissolved moisture with the resulting tendency for freezing of the exhaust ports in the machine. It has been reported that in efficient machines the temperature of the air can drop as much as 70.degree. F. during passage through the machine. Freezing of the exhaust, of course, renders the machine incapable of further use when the exhaust ports become blocked with ice and the machine cannot be used again until the ice is cleared from the exhaust ports. The action of a miner in clearing the exhaust is all too often highly drastic in nature. Assaults on the machine with hammers, picks, drill steel, wrenches and the like are all to common when freezing of the exhaust occurs. Such assults can and frequently do result in breakage or permanent damage to the air cylinder. In addition, since the air operated drill is energetic, noise is generated, especially when operating underground, at a level sufficiently high to be physically damaging and/or painful. Governmental regulations now frequently require that each pneumatic drill be provided with a muffler which is usually welded or brazed directly on the body of the machine so as to cover the exhaust ports with the muffler being provided with exhaust holes at an end thereof. Such mufflers do offer some sound attenuation. However, the problem of exhaust freezing still remains and the extent of sound attenuation is insufficient. Steel mufflers of the aforedescribed type which are welded directly to the body of the air cylinder are roughly rectangular in shape with a flat outer face standing free of the air cylinder and with extending sides which are welded to the air cylinder so as to form a box enclosing the exhaust ports of the machine itself. It is desirable that freezing of the exhaust ports be prevented altogether and that further attenuation of sound level be achieved.
Many approaches to the foregoing problem have been suggested in the prior art. Thus U.S. Pat. Nos. 3,815,705, 3,554,316, 3,365,022, 4,010,819, 4,079,809, and U.K. Pat. No. 329,239 disclose devices intended to solve the problem. While the aforementioned devices have offered some improvement in performance, the resulting performance is still not acceptable from the operating viewpoint. Air operated feed-leg drills commonly used underground are called respectively a "stoper" and a "jackleg drill". As is known, the stoper is elevated from the ground by means of an air cylinder which is aligned with the drill body. The jackleg drill, on the other hand, has an air cylinder at an angle to the drill body and is fastened thereto by means of a swivel joint. The function of the elevating air cylinders is to press the drill steel against the bottom of the hole being drilled. It is also to be remembered that a conventional penumatic drill has a reciprocating air motor which is relatively small in size and the space in which a muffler can be mounted is limited by the dimensions of the air cylinder itself. Thus a muffler to be acceptable to an underground miner must not interfere with operation of the machine by the miner himself. Furthermore, due to the nature of drilling work underground, it is not practical to isolate the machine from the surrounding as can be done, for example, by building a soundproof room around a noisy device. Instead, the miners must work in close proximity to the drill while it is operating. These factors combined with the high level of sound intensity generated by the pneumatic drill in action makes the problem of providing a satisfactory muffler a difficult one indeed. The air cylinder reciprocates at a rate of approximately 2000-2400 cycles per minute generating an exhaust of high velocity air, and an exhaust noise having a wide range of sound frequencies.